1. Field of the Invention
The invention relates to emitter follower outputs. More particularly, the invention concerns an integrated circuit having emitter follower outputs and an electrical connection area that is connectable to an external circuit to program the amount of current in the emitter follower outputs.
2. Description of the Related Art
Emitter follower output stages are common components of digital logic circuitry. FIG. 1 illustrates a typical prior art buffer circuit 100 that has emitter follower outputs. In the circuit of FIG. 1, the emitter of output transistor Q1 is connected to output Y, and the emitter of output transistor Q3 is connected to complimentary output YN. The output current I101 for output Y is set by a constant current source consisting of transistor Q2 and resistor R1. Similarly, the output current I102 through output YN is set by a constant current source consisting of transistor Q4 and resistor R2. Output currents I101 and 102, which are also referred to as output drive currents, primarily affect voltage fall times.
When an integrated circuit including a circuit such as circuit 100 is designed, the output currents of the circuit 100 are essentially fixed, except for resistor sheet resistance variation. Once a circuit such as circuit 100 is fabricated, the output currents are fixed. The bias voltage applied to the bases of current source transistors Q2 and Q4 is constrained to a range of values, which sets the output currents of the emitter follower outputs at fixed values.
The fixed output currents in emitter follower outputs such as the outputs of the circuit 100 in FIG. 1 are typically set high enough to ensure satisfactory performance for a wide range of applications under worse case conditions, also taking into account imprecision in the output currents that may result due to manufacturing process tolerances. For example, the output currents are set high enough to adequately minimize propagation delay times and setup and hold times, and to maximize "eye openings," in a variety of applications. A significant shortcoming of these prior art emitter follower output circuits is that many applications do not require output currents that are as large as the output currents provided by these circuits. Consequently, in many applications a substantial amount of power is wasted and a corresponding amount of heat is needlessly generated due to the excessive output currents provided by these circuits. Conversely, some applications may require output currents that are greater that the fixed output currents that are provided.